1. Field of the Invention
This invention relates to a novel process for producing a polyol ester composition which possesses a desirable composition and color.
2. Related Art
During the past few decades, water-borne coatings have found broad acceptance in architectural as well as industrial applications and are expected to have continued good growth rates in the coating industry. The driving forces behind this trend are based upon both environmental and economic concerns to reduce volatile organic compounds (VOC) of most coating materials. Traditionally, latex coatings, based upon small particles (emulsions) of a synthetic resin such as acrylic polymers, have required the use of a coalescing aid in substantial quantities. The coalescing aid in latex coatings is added to improve the filming properties of the coatings. The function of the coalescing aid is to soften the latex particles so they can flow together and form a continuous film with optimal film properties after the water has evaporated. Without the coalescing aid, the latex coatings may crack and not adhere to the substrate surface when dry at ambient temperatures.
Alcohol esters and ether alcohols, such as ethylene glycol monobutyl ether and TMB (2,2,4-trimethyl-1,3-pentanediol monoisobutyrate), are widely used as coalescing aids in the latex coatings industry. These chemicals are volatile and are counted as VOC in the paint.
Although newer formulations of water-borne paints and coatings have reduced the amount of coalescing aids, they have not eliminated them. It is estimated that the market for latex paint coalescing aids is on the order of 120 million pounds in the U.S. and perhaps twice that amount globally. Currently, nearly all of this material is lost into the atmosphere each year. Proposed regulations have limited the level of VOC in latex paints even further.
Recently, glycol esters of unsaturated fatty acids (such as the propylene glycol monoesters (PGME) of vegetable oil fatty acids) have been shown to perform in the same way as TMB to cause the latex particles to coalesce and form a cohesive film (N. Jiratumnukul & M. Van De Mark, JAOCS, vol. 77, no. 7, 691, 2000). Due to the large relative size (molecular weight) of these unsaturated esters, these coalescing aids are essentially, non-volatile, and therefore do not contribute to the VOC of the coating. Instead of evaporating into the air over a few days as in the case of conventional coalescing aids, these glycol esters oxidize and remain within the film. Over the course of a few days the coating dries and becomes harder and more durable without releasing VOC into the atmosphere. It is important that the fatty acids be primarily polyunsaturated fatty acids to facilitate this oxidative reaction. The primary coalescing aid is propylene glycol monoesters (PGME) of polyunsaturated fatty acids.
The effect of PGME coalescent on yellowing in the paint film is one concern. Another issue is the visual color of the liquid PGME product to be used. To compete in the paint industry with TMB (a color-less water-clear liquid), a clear, almost colorless liquid PGME is needed. Color can be measured using a Lovibond tintometer, which is an instrument for evaluating oil colors on the Lovibond scale (a standard scale in the industry). The color of a monoester mixture can be determined by comparing the monoester mixture product to standard reference samples.
In general, the preparation of fatty acid esters of polyols (such as propylene glycol) is possible from a number of routes. For example, propylene glycol and triglycerides can be reacted together using an alkaline catalyst to give a reaction product comprising monoesters of propylene glycol, propylene glycol diesters, monoglycerides, diglycerides, and triglycerides, after removal of the excess propylene glycol and glycerol (Hui, Y. H., “Manufacturing Processes for Emulsifiers” in Bailey's Industrial Oils and Fat Products, John Wiley & Sons, Inc.(1996) 5th Ed., Vol. 4, pp. 569–601).
A second route is through the reaction of propylene glycol with fatty acids or fatty acid esters, such as methyl or ethyl esters of fatty acids (Swern, D., “Fat Splitting, Esterification, and Interesterification” in Bailey's Industrial Oils and Fat Products, John Wiley & Sons, Inc.(1982) 4th Ed., Vol. 2, pp. 97–173). An acid such as para-toluene sulfonic acid catalyzes the esterification of palmitic acid and propylene glycol (U.S. Pat. No. 3,669,848). Reaction of fatty acid methyl esters with glycols was accomplished with the addition of metallic sodium as catalyst and the evolution of methanol (R. Celades and C. Paquot, Chem. Phys. Appl. Surface Active Subst., Proc. 4th Int. Congr. (1964; published 1967) 1, 249–255). The product from these reactions will generally be a mixture comprising primarily mono- and diesters of propylene glycol after the removal of water or the low-boiling alcohol (ethanol, methanol, etc.), byproducts and any excess starting reactants.
A third route is combining propylene oxide with fatty acid, leading to a mixture of monoester isomers.
A fourth route is combining propylene glycol with an acid chloride of a fatty acid.
The final product composition of these processes can be described in terms of the ratio of mono- to diesters comprising the product. The composition of the end product can be controlled by varying the amounts of glycol with respect to fatty acid reagent (methyl ester, fatty acid or oil), and through manipulating the reaction conditions. The above reaction processes, however, consistently generate color during the preparation of the propylene glycol fatty acid ester product.
If a very high propylene glycol monoester product was desired, in general, a distillation process was the most widely used technique for such purification. Due to the high molecular weight of the propylene glycol fatty acid esters, an expensive subsequent molecular distillation was required to obtain higher monoester concentration. The crude monoester mixture is distilled under high vacuum, in a short path distillation process. The distillate generally comprises greater than 90% (by weight) monoesters. The remaining material generally comprises mainly propylene glycol diesters, monoglycerides and/or diglycerides, depending upon the starting reactants used. Previously, this molecular distillation was the only method which yielded a nearly colorless propylene glycol monoester product of unsaturated fatty acids.
As described above, it is desirable to prepare a propylene glycol fatty acid monoester mixture of acceptable color for use as a non-volatile coalescing aid for latex paints. A dark-colored monoester mixture is not suitable for incorporation into coating compositions. It would be useful to develop a process for producing propylene glycol monoesters of polyunsaturated fatty acids in which the product of said process contains a high amount (at least about 85%) of monoester and has the desirable characteristic of a light color, without the need for purification through a molecular distillation process or a decoloration step.